1. Field of the Invention
This invention relates to a means and method for administering two physiologically active gases, typically oxygen and an anesthetic, to a patient.
More particularly, this invention relates to a system for delivering variable doses of two gases at variable ratios of one gas to the other in response to a patient's breathing cycle and to a method for use of that system.
It is common to employ gases as anesthetics or analgesics in surgical procedures especially in dentistry. Gas administration devices used for this purpose typically provide a system for mixing oxygen with an anesthetic gas, usually nitrous oxide, and require two independent and continuous flow systems, one for each gas. The ratio of oxygen and anesthetic gas is varied during the procedure in order to control the level of sedation. It is of critical importance that an adequate level of oxygen be supplied to the patient with every breath and that the anesthetic gas be supplied only in admixture with oxygen else harm to the patient may well result.
The mixture of oxygen and anesthetic or analgesic gas is typically administered to the patient through a cannula or nasal mask as a continuously flowing stream. In a typical breathing cycle, the patient is inhaling for only about one-third of the time with the remainder of the cycle taken up by an exhalation and a pause after exhalation is complete. Thus, devices which continuously furnish oxygen and an anesthetic gas mixture result in wasting two thirds or more of the total gas supplied. While this wastage does have considerable economic significance, more importantly, there tends to result a buildup of physiologically active gases about the patient. Even with careful ventilation, there exists the danger that medical personnel attending the patient will be affected by the anesthetic. This hazard is particularly pronounced in dental surgery.
2. Description of the Prior Art
Applicant's prior U.S. Pat. No. 4,705,034 describes an inhalation responsive system for dispensing metered doses of two different gases. The system uses a pair of displacers, one for each gas, and includes means for varying the ratio of one gas to the other.
U.S. Pat. No. 4,457,303, to Durkan, discloses a respirator system for supplying supplemental oxygen to a patient which uses a fluidic laminar proportional amplifier to sense the start of an inspiration. Rate metered oxygen flow is started in response to the sensed inspiration and flow is continued for a time period shorter than the inspiration period.
Myers in U.S. Pat. No. 4,054,133 and Mon in U.S. Pat. No. 4,381,002 disclose devices which sense inhalation and exhalation pressure in the nasal cavity of a patient and convert those sensed pressures to signals which control the flow of oxygen to a patient.
None of the known prior art devices suggest a displacement type, inhalation responsive, two-gas variable ratio, variable dose, delivery system as described and claimed in this application.